This invention relates to welding small oxide coated components together, for example, welding wires, ribbon, studs, pellets and wafers of the same or different oxide coated metal.
Two processes have developed over the years for welding small parts together. In alternating current resistance welding, the parts are pressed together and an alternating electrical current is applied across the joint. The alternating electrical current heats the parts and especially the joint and smaller part, say wire, until the two parts fuse together. In this process, sparking and/or arcing is observed when the alternating electrical current is applied. An example of apparatus for carrying out the resistance welding process is shown in Pityo U.S. Pat. No. 2,644,069. In this prior art patent, it is taught that as soon as the two parts abut a switch is closed either automatically or manually. Stepped up alternating current is then applied across the joint for a period of time which is adjustable and controlled by a suitable timer. When the timer times out the alternating current is interrupted and the welded part removed. This process is relatively slower than the percussive welding process next to be described. It is useful with metals that are easily ignited and in fact was the only process that could practically be used for welding tantalum wire to tantalum studs. Unfortunately, the time period for welding tantalum wire to tantalum studs using resistance welding is long due to the high resistance oxide coating that immediately forms on tantalum exposed to the air. The high resistance so restricts the alternating electrical current that it is only possible to weld parts at a rate of about one every two or three seconds. Even at this rate, the welds are not sufficiently reliable.
In the so-called percussive welding process, the mechanical apparatus is similar to that used for alternating resistance welding except the parts are driven together faster and with greater force. The electrical circuitry differs substantially. In this process, a charged capacitor is connected across the wire and stud, for example, before the parts are driven together. First a spark and then an arc are generated while the pieces are driven together. The spark is obtained by either first striking the two parts and then through an appropriate mechanism separating them to enable an arc to flow or by ionizing the space between the parts prior to contact. Initially, the wire may melt back away from the joint faster than it is being driven into the other part. This may be the mechanism for separating the parts when the arc is initiated by striking of the parts. The welded joint may be characterized by a weld fillet tapering away from the wire to the stud surface. There may be a certain amount of splashing and spattering of molten metal back over the pellet and wire. An electrical circuit for percussion (arc) welding is ilustrated in Frank U.S. Pat. No. 2,755,365 in which a very high frequency signal is imposed on the circuit to aid in initiating the spark and then arc. Frank teaches that the prior art relied upon the proximity of the two parts to time the initiation of the arc. By use of a high frequency signal arc initiation can be better controlled independent of the position of the parts. Frank suggests that the factors affecting the success of the percussion (arc) welding process are (1) the timing between the mechanical and electrical phases, (2) the intensity of the discharge (3) the duration of the arc, and (4) the amounts of percussive force. Frank teaches that it is desirable with the percussion welding process to start the arc soon enough, that is, while the parts are sufficiently spaced as the arc is necessarily extinguished on contact. Moreover, Frank teaches that it is desirable to maintain the arc until contact. Otherwise, it is taught, a good weld is not achieved. Frank teaches the arc may exist from 100 to 500 microseconds. The teachings of the Phillips et al. U.S. Pat. No. 3,654,423 are much the same as those of Frank. It is taught to use a high frequency signal to initiate the arc and that the arc discharges capacitors charge to 1600 volts. Phillips et al. claim to provide exact control over the (1) arc energy, (2) arc duration, and (3) the timing of triggering of the arc. Phillips et al. teach that the total amount of energy stored in the capacitors is adjusted to be substantially discharged and converted to thermal energy by the arc prior to the time the two parts contact. Reutschi U.S. Pat. No. 3,505,494 teaches drawing out the burning time of the arc by drawing the two parts together with apparatus that allows the arc itself to hold the two parts separate until the arc is extinguished. Peterson U.S. Pat. No. 3,433,921 teaches that at least 50% of the energy initially stored on the capacitor should be dissipated in the arc during the burn back interval.
As will become apparent, the applicants' process is neither an alternating electrical current resistance welding process nor a percussion (arc) welding process wherein the arc commences prior to collision or contact. The applicants' process involves the discharge of capacitively stored energy through the joint of the two parts being welded but with every attempt to minimize the intensity of the arc. Whereas the weld characterized by the percussion (arc) process described in the prior art patents is a fillet or skirt surrounding the wire part, no such fillet is produced by the applicants' process. The absence of the fillet is evidence of a different physical mechanism of welding. Moreover, photomicrographs of sections of welds according to this invention illustrate the wire part actually penetrates the other part, the degree depending on the hardness of the materials.
The very property of certain metals such as aluminum, magnesium and tantalum that makes them easily ignited by a hot arc (thus not suitably and safely welded by the percussion (arc) process) makes them develop an electrical resistive oxide coating (thus making it difficult to weld them with the alternating electrical current resistance welding process). Applicants do not wish to be tied to any particular theory explaining the success of the process and apparatus described herein to provide excellent welds with these difficult materials. However, it appears that just after the instance of touching, the oxide layer is sufficiently disrupted by the mechanical collision and/or the electrical field and arcing, such that rapid discharge of the stored electrical energy takes place very near the surface and possibly surrounded by a protective oxide plasma. Electrical arc and resistance heating raises the temperature of the parts especially a wire and the joint enabling a fusion of the two together without the large fillet characterizing some arc welds. Another aspect of the mechanism of welding according to this process is the minimization of burn back prior to contact coupled with the shaping of one part to a point and driving of the point into the other part enabling a wire, for example, to first contact and then slowly burn back allowing the arc to be maintained long enough to fuse the surface of the other part such that the wire can penetrate deep into a pellet, for example, during the discharge of the electrical welding energy.
One costly and difficult step in the fabrication of tantalum capacitors has been the attachment of the porous tantalum anode pellet to a solid tantalum lead wire. In many cases, this attachment has been made by molding and heating tantalum powder around the solid wire. Attempts to produce a more reliable electrical and mechanical connection between the anode and the lead wire have led to the use of manual control of resistance welding methods. Though that has been reasonably successful, a machine capable of high production rates has never been developed. The welder described herein can easily make an eight-to-one productivity improvement over manually operated machines currently used for the welding of tantalum capacitor anodes. Moreover, the applicant is unaware of even claims to weld aluminum wires by any process. Both tantalum and aluminum wire can be rapid welded according to this invention. For example, it is now possible to weld tantalum wire to sponge tantalum pellets at a rate of 12,000 welds per hour.